Spread spectrum clock generation for the reduction of radiated emissions

A method is presented for reducing the radiated emissions of an electronic device by frequency modulating (FM) the system clock. This method, referred to as spread spectrum clock generation, or SSCG, is applicable to most microprocessor based systems. A unique waveform used to frequency modulate a digital clock signal results in a spectrum with sideband harmonics that are nearly uniform in amplitude when measured with an EMI receiver. This has the effect of spreading the energy of a discrete frequency harmonic over a wider bandwidth, thereby reducing the amplitudes of the harmonics. Attenuation as high as 13 dB is presented using an experimental setup, an actual SSCG integrated circuit, and theoretically computed results